Polyvalent metal salts of alkyl-substituted salicylic acids, in particular zinc salts, have been used as color developers in pressure-sensitive recording paper.
In Japanese Patent Publication No. 25174/1976, polyvalent metal salts of salicylic derivatives which are disubstituted in positions 3 and 5 by an .alpha.-methylbenzyl, .alpha.,.alpha.-dimethylbenzyl or cyclohexyl radical are mentioned as organic color-developers for pressure-sensitive recording paper. In the U.S. Pat. No. 4,748,259, a mixture of the zinc salt of 5-[.alpha.-methyl-4'-(.alpha.-methylbenzyl)benzyl]salicylic acid and zinc salt of 3,5-bis(.alpha.-methylbenzyl)salicylic acid, and in a 50/50 weight ratio, both of which have two styrene units, is disclosed as a color developer. In Japanese Patent Application Laid-open No. 91043/1990, a mixture of 3,5-bis(.alpha.-methylbenzyl)salicylic acid, 3-.alpha.-methylbenzyl-5-(1,3-diphenylbutyl) salicylic acid, 5-.alpha.-methylbenzyl-3-(1,3-diphenylbutyl) salicylic acid which have two or three styrene units in a weight ratio of 72:28 or 85:15 is still disclosed.
However, developers containing the above salts are insufficient as to both their speed of color development and their color concentration; in other words they provide inadequate image density particularly when multiple copies are required in rapid succession.
They also discolor easily, and are particularly susceptible to yellowing when exposed to NOx gas.